476 SCIENCE PROGRESS. 



Rosenbusch (16) has adopted an important distinction 

 between the even-grained {k'brnig) structure, in which each 

 constituent mineral belongs to a single stage in the con- 

 solidation, and the porphyritic, in which there is a re- 

 currence of one or more minerals in a second generation, 

 so that the constituents belong to two periods, the " intra- 

 telluric " and the " effusive ". The two types of structure 

 are typically exemplified in the plutonic and the volcanic 

 rocks respectively. On this point the German and French 

 schools are at variance, the latter claiming to recognise two 

 distinct periods of consolidation in almost all igneous rocks, 1 

 but on examination the difference seems to be one of 

 theoretical rather than practical importance. A more real 

 difference of opinion exists as to the propriety of establish- 

 ing an intermediate division between the plutonic and the 

 volcanic types of structure. Such a division is roughly re- 

 presented by the " dyke-rocks " of Rosenbusch (22, 36), who, 

 however, does not carry out the principle, systematically. 

 The difficulty arises chiefly with reference to the basic 

 rocks, the strong tendency to crystallisation of magmas 

 poor in silica rendering the structural and textural char- 

 acters in their case of less significance than in the more 

 acid rocks. Brogger (35) has insisted upon the necessity 

 of a division intermediate between the plutonic and the 

 volcanic, which he terms " hypabyssal," the distinction being 

 based practically upon structural characters ; while Roth, 

 Zirkel, and Lossen, as well as the French writers, have 

 considered such a division superfluous and ill defined. 



It is manifest that, if we are to work towards a true 

 natural classification, we must have regard in the first place 

 to characters which are original and essential. This brings 

 us to another question on which unanimity has not yet been 

 attained. Allport, Bonney, Rutley, and other English 

 petrologists have long maintained that many fine-textured 

 rocks, especially among those of acid composition, owe their 

 present structure to the secondary devitrification of a mass 



1 For further criticism of Rosenbusch's views on this point see Zirkel 

 (34), and especially Cross, Fourteenth Ann. Rep. U.S. Geo/. Surv., p. 233, 

 1895- 



